Rogowski coils measure magnetic fields. Early Rogowski coils could not be used for current measurements since the coil output voltage and power were not sufficient to drive measuring equipment. With the introduction of microprocessor-based protection and measurement equipment, Rogowski coils have become more suitable for use in such equipment. Prior to the advent of microprocessor-based equipment, current transformers (CTs) had been used for protection and measurement applications in part because of their ability to produce the high power output needed by electromechanical equipment.